1. Field of the Invention
The present invention relates to a fluorescent substance, a manufacturing method of a fluorescent substance, and a light-emitting device using the fluorescent substance.
2. Description of the Related Art
An LED lamp using a light-emitting diode is used in various display devices including, for example, a portable device, a PC peripheral device, an OA device, various switches, a light source for the back light, and a display board. The LED chip, which is a semiconductor element, has a long life and exhibits a high reliability and, thus, when used as a light source, the replacing work can be reduced. Such being the situation, the application of the LED chip to various uses is being attempted.
The known LED lamp includes a lamp using in combination a light-emitting diode used as an exciting light source and a fluorescent substance. It is possible for the particular lamp to emit light of various colors depending on the kind of the combination noted above. In order to obtain a so-called white LED, i.e., a light-emitting device that emits a white light, known is the combination of a light-emitting diode that emits light having a wavelength of 360 nm to 500 nm, e.g., a light-emitting diode having a GaN-based compound semiconductor layer, and a fluorescent substance. In the above combination, known are a combination of a blue-light-emitting diode and a yellow-emitting fluorescent substance and another combination of a near-ultraviolet light-emitting diode, a blue-emitting fluorescent substance, a yellow-emitting fluorescent substance and a red-emitting fluorescent substance. Incidentally, the term “near-ultraviolet light” noted above denotes an ultraviolet light having a wavelength of 360 nm to 410 nm. As described above, the fluorescent substance used in the LED lamp is required to absorb efficiently the near-ultraviolet light to blue light having a wavelength of 360 nm to 500 nm and to emit also efficiently the visible light.
A yellow-emitting YAG:Ce-type fluorescent substance is known well as a fluorescent substance used in the white LED. However, when excited by the near-ultraviolet light having a wavelength of 360 nm to 410 nm, the luminescence efficiency from the YAG:Ce-type fluorescent substance is low. Therefore, the application of the YAG:Ce-type fluorescent substance is limited to the combination with a blue LED. Also, the emission spectrum of the YAG-type fluorescent substance has a wide emission spectral band and, thus, the emission efficiency as a white LED is limited.
A divalent europium-activated silicate fluorescent substance, i.e., (M, Eu)2SiO4 (M representing an alkaline earth metal) is known as a fluorescent substance that emits luminescence of colors ranging from green to yellow or orange when excited by the near-ultraviolet light or the blue.
Already proposed as a silicate fluorescent substance for LED is (M, Eu)2SiO4 (M representing an alkaline earth metal), where Si≦1.00. The properties of the silicate fluorescent substance excited by an ultraviolet light having a short wavelength of 254 nm have already been examined. It has been reported that, where the Si amount is larger by only 0.5% than the stoichiometric composition, the emission efficiency is lowered to about 50% of that of the stoichiometric composition.
The light-emitting device utilizing the combination of the conventional silicate fluorescent substance and the LED is required to exhibit a higher emission output. The present invention is intended to provide a fluorescent substance that makes it possible to obtain a light-emitting device of a high output, to provide a manufacturing method that makes it possible to manufacture the particular fluorescent substance with a high stability, and to provide a light-emitting device of a high output.